Basically I'm wondering why my 2 bodies vary so much in terms of blowing small amounts of highlights when using flash. I shoot a 7D and a 5D2 - both with equivalent lenses (10-20 and 17-40) and use the same flash (430ex). When using an on camera flash the 5D is fine, the 7D however blows highlights regularly (normally on people's foreheads which are closest). The exposure is right on both cameras but the 7D is a pain as I have to recover the highlights. My question:

Is the dynamic range on the 7D really that much worse than the 5D2 for practical applications like this? Or is something else causing this?

I'm genuinely considering dumping the 7D for another 5D2 if this is the case - only thing that is making me keep it is the 50fps video

EDIT: I realise I might not have given enough information. To clarify:
I am shooting with identical settings on both cameras with the exact same flash at the same distance. The images are practically identical in terms of set up. I am shooting in manual mode with ETTL flash.

My 5d2 always needed + FEC and my 7d doesn't need it so much - which probably leads to your issue. Dial the FEC down a little, and you'll probably be fine. There's also the question of which exposure mode you have the camera set to for flash (evaluative vs center weighted)

The 7d's a really good complement to the 5d2 - I shot that pair for 3+ years of weddings; now use 5d3 + 7d

dialling down the FEC is an option but that lowers the exposure of the entire image - something I don't really want. I just want to retain detail in the highlights which the 7D seems incapable of doing but the 5D does.

I shoot with both the 5D2 and 7D2 and, yep, use an ancient times 430EX. I like ambient light with flash understated (mellow fill) and find the 7D meters slightly hot and thus I tend to use -1/3 to -2/3 FEC. I rarely touch FEC on the 5D2 as it nails it most of the time. The 7D also seems to get fooled by dark clothing more easily than the 5D2 but these behaviors are very consistent so I don't think about it much anymore. Just diddle the dial as needed.

I think it's a dynamic range thing. I've been trying to photograph a train in a well lit covered station with daylight at the far end. With my eye I have no trouble seeing details outside (heavy overcast day) and along the station roof and in all the shaddows. But the 7D image has the open section blown out and, the roof pitch black. I find that there are no details recorded to recover any of this. I think I might go and hire a FF camera and see if I can be more successful. It could also be that I'm at ISO6400, 1/800th @ f3.2, the train is moving, and I'm just pushing the envelope too far.

Ferrophot wrote:
It could also be that I'm at ISO6400, 1/800th @ f3.2, the train is moving, and I'm just pushing the envelope too far.

Yep, ISO 6400 is useable for fairly even lighting, sports in ice rink, but will suffer badly in high contrast situations. Dynamic range contracts with each stop of higher ISO. For my taste, ISO 1250 is my limit and even that needs careful NR. My fav high ISO on the 7D is 320 and 640.

Langran wrote:
dialling down the FEC is an option but that lowers the exposure of the entire image - something I don't really want. I just want to retain detail in the highlights which the 7D seems incapable of doing but the 5D does.

Yep, that's right, dialling down the FEC will lower the exposure of the whole image. If you want to avoid blowing out highlights, you don't have any choice about this, you will have to accept reduced flash output across the entire image.

Basic physics I'm afraid. A flash can only put out one burst of light, it can't magically send reduced output to certain areas of the frame (unless you add a modifier to the flash or use multiple flashes to give you finer control)

There are 2 possible causes for the difference you are seeing between the 7D and 5D2
1. Dynamic range. The 5D2 has a full frame sensor and has better dynamic range. Hence better detail retention in the highlights. Reducing the ISO will give you more dynamic range from your 7D sensor.
2. Flash control. The 5D2 may be 'better' at controlling the flash via ETTL. From what you have said about not being able to fix it by dialling in FEC, this isn't the issue.

Langran wrote:
dialling down the FEC is an option but that lowers the exposure of the entire image - something I don't really want. I just want to retain detail in the highlights which the 7D seems incapable of doing but the 5D does.

FEC only lowers the flash output. Ambient light is unchanged. If you're balancing ambient with fill flash, lowering flash output normally imparts a more natural look. On the other hand, if you are shooting with the flash as the main light--pitch dark nightclubs--you can't reduce flash output much. However you can reduce forehead hotspots by bouncing off a wall/ceiling, direct flash with a wee soft box or even using the flash on a bracket or in your hand.

thanks for the replies. I guess it must be a dynamic range issue. I am proficient enough to know when my exposure for skin tones and clothing etc is good but I am blowing small spots. Lowering the FEC would get rid of the bright spots but would lead to an underexposed looking image and needing to in turn recover shadows. I understand why it can happen - I'm just surprised that the 7D is as bad at dealing with it in comparison to the 5D2

Gochugogi wrote:
if you are shooting with the flash as the main light--pitch dark nightclubs--you can't reduce flash output much. However you can reduce forehead hotspots by bouncing off a wall/ceiling, direct flash with a wee soft box or even using the flash on a bracket or in your hand.

you hit it on the head there. I'm working a lot in night clubs and despite bouncing where possible and using an omnibounce to aid this I'm still getting the hot spots on the 7D.